Currently telecom operators upgrade their Radio Network Subsystem (RNS), i.e. Radio Network Controller (RNC) and Radio Base Stations (RBS) at separate times and this happens generally during different maintenance windows (mostly at nights). When the RNC is being upgraded it does not handle any calls. Similarly, when the RBS is being upgraded it does not handle any calls.
In order to minimise the impact of such software upgrades on the operation of the network the RBSs are upgraded in batches. There are many ways to select batches of RBSs for upgrade. One, commonly used, is based on RBS type because of similar upgrade times.